The prior art in this field is voluminous, as considerable effort has been dedicated over the years to the development of improved methods of fabricating metal structural components. For the purposes of the present invention, related prior art can be classified generally into three groups, as described herebelow.
First, it has been recognized in the past that metal structural elements can be made with less metal by eliminating part of the material in areas of lower stress such as the web. Examples of structural members having weight reduced areas are U.S. Pat. No. 3,812,558 (issued on May 28, 1974) to Watanabe, entitled "Method and apparatus for manufacturing expanded structural members and its products"; U.S. Pat. No. 4,418,558 (issued on Dec. 6, 1983) to Simmons entitled, "Method of manufacture of ventilated sheet metal floor members"; U.S. Pat. No. 5,551,135 (issued on Sep. 3, 1996), entitled "Method of fabricating a metal purlin and method of fabricating a building therewith"; U.S. Pat. No. 5,661,881 (issued on Sep. 2, 1997) to Menchetti, entitled "Method of making framing components of expanded metal"; U.S. Pat. No. 5,778,626 (issued on Jul. 14, 1998) to Hellsten, entitled "Closed beam with expanded metal sections".
The principle disadvantage of the approaches illustrated by these patents is that the high stress areas such as flanges or edges are not formed separately from the expanded material. These areas are constructed as an integral part of the parent metal of the structural element. This means that expensive and complex equipment is required to expand the required areas of the element thereby increasing the cost of production. This factor also limits the design of the structural element because the thickness and material of the element in the high stress areas is the same as that used in low stress areas. These disadvantages may explain why this approach to building structural components has never gained much commercial popularity though it has been studied for many years.
Secondly, where edge pieces have been applied to expanded metal to form useful objects, they have been used as frames to enclose and stiffen the expanded metal sheet and have been developed for very specific applications. Examples are, U.S. Pat. No. 3,583,100 (issued on Jun. 8, 1971) to Catalano, entitled "Framed panel"; U.S. Pat. No. 4,955,125 (issued on Sep. 11, 1990) to Steinman, entitled "Method of forming a pizza grille"; U.S. Pat. No. 5,787,642 (issued on Aug. 4, 1998) to Coyle, entitled "Storm shutters with light transmittance".
Among these patents, U.S. Pat. No. 4,955,125 has a donut shaped disc applied around the edge of a circular piece of expanded metal to form a pizza grille. The donut shaped disc is then formed into a "U-shaped" edge piece in order to stiffen the expanded metal and protect the user from any sharp edges. The development of the approach illustrated by these patents has not been expanded to the use of framed strips or sheets of expanded metal as base elements to be formed and assembled into more complex structures. Each of these approaches has a very limited scope and the products are adapted for a specific function.
Thirdly, there have been several patents with approaches to creating or applying structural flanges to webs. Examples are, U.S. Pat. No. 5,403,986 (issued on Apr. 4, 1995) to Goleby, entitled Structural member and method of making by cold rolling followed by induction or resistance welding; and U.S. Pat. No. 4,246,737 (issued on Jan. 27, 1981) to Eiloart, entitled Metal structural members.
The above-mentioned U.S. Pat. No. 5,403,986 discloses hollow flanged structural members created by continuously rolling strips of sheet metal in parallel with the web to form hollow flanges. These flanges are then continuously welded to the web. This patent discloses a beam produced using different materials in high stress areas, however the manufacturing process is complex and therefore would be expensive to produce.
Another example is U.S. Pat. No. 4,246,737 to Eiloart where a flat strip of metal is rolled into an "I-beam" shape by folding the edges of the strip to form flanges and then closing the folded assembly with welds or fasteners. This has the aforementioned difficulties of the material being used in the high stress areas being the same as that in the low stress areas.
None of the above patents employs separate edge pieces assembled together with a lightened central web to be used as a structural element. Rather, those prior art devices discussed above use edge pieces as framing devises only. Further, the above patents do not show a structural element that includes separate edge pieces assembled together with a lightened central web that is bent to form a three-dimensional structure.
Furthermore, the prior art in the field of industrial platforms or pallets is voluminous, as over the years considerable effort has been dedicated to the improving methods for fabricating pallets and industrial platforms. Pallet designs break down into three general groups: namely, grid type pallets, sheet pallets, and wire-form pallets.
In grid type pallets, the structural elements are arrayed into some form of an orthogonal grid or network of spaced ribs or members. Examples are U.S. Pat. No. 5,687,653 (issued on Mar. 15, 1995) to Bumgarner, entitled "Modular metal pallet"; and U.S. Des. Pat. No. 335,743 (issued on Oct. 9, 1991) to Nordstrum, entitled "Loading pallets". The main disadvantage of grid type metal pallets is their great weight. This is why the most popular pallets are manufactured from wood. With a metal construction, weigha disadvantage not only because of the higher transportation cost, but also because of the high production cost.
In wire-form pallets, metal tubing or wire is shaped into co-planar surfaces in such a manner as to support heavy loads and provide open spaces through which to insert the forks of a fork lift truck. Examples are U.S. Pat. No. 3,756,167 (issued on Sep. 4, 1973) to Wilson, entitled "Wire-formed Pallet"; and United Kingdom Patent No.1,587,993 (issued on Apr. 15, 1981) to Dipalma, entitled "Improvements Relating to Material Handling Pallets". One of the disadvantages of wire as a construction material is the high cost of wire products both to buy and to manufacture relative to sheet products. The above-mentioned U.S. Pat. No. 3,756,167 is a hybrid design but has the disadvantage of poor torsional strength. Low torsional strength results in a short working life due to the high stress and load cycling placed on the connections.
In sheet pallets, sheets of metal or other material are formed into various shapes to form the pallets or industrial platforms. Sheet pallets are generally classified into two sub-groups, either single or multiple sheet construction depending on whether a single sheet of material is used to form the pallet or two or more sheets are assembled to form the pallet. Sheet pallets include the large group of pallets designed to be formed from plastic by injection molding but whose shape is similar to the shapes of deformed sheet materials. Sheet pallets also include pallets formed from corrugated paper sheet material. An example of a pallet of single sheet construction is Canadian Patent No. 961,351 (issued on Jan. 21, 1975) to Morrison, entitled "Pallet and Method of Production".
Examples of pallets of multiple sheet construction are: U.S. Pat. No. 4,240,360 (issued on Dec. 12, 1980) to Sanders, entitled "Stackable Flat Pallet" and U.S. Pat. No. 5,460,103 (issued on Feb. 17, 1992) to Dunn, entitled "Metal Pallet".
The above-mentioned U.S. Pat. No. 4,240,360 to Sanders employs an orthogonal grid of metal strips to support a pierced sheet metal deck that enables the pallets to be stacked. This design however is heavy due to the weight of the full metal sheet used for its deck. The above-mentioned U.S. Des. Pat. No. 335,743 (issued on May 18, 1993) to Nordstrum uses a deck constructed from metal strips. However this design exhibits poor torsional stiffness. This is a major operational drawback as pallets are often heavily loaded and are not treated carefully in operation.
Therefore, it would be advantageous to provide a structural assembly that is more dimensionally stable, thereby enabling easier handling and easier assembly in a variety of ways at a lower construction cost. In addition, it would be advantageous to provide a load-bearing structure that has much greater torsional stiffness and strength than the prior art designs while maintaining a relatively light weight.